This invention relates to a semiconductor light-emitting device, and more particularly to a light-emitting diode (LED), preferably used for optical communication, in which the launching efficiency, that is, the ratio of the quantity of radiation transferred through an optical fiber to the total quantity of radiation emitted from the light-emitting portion of the LED, is increased.
In general, since the refractive index of crystal material used for fabricating an LED is very high, the critical angle at a plane surface of the crystal, that is, the minimum angle for total reflection, is very small. Therefore, only a small quantity of light emitted from the LED can be obtained outside the crystal. For example, when the crystal is made of Ga.sub.1-x Al.sub.x As at a refractive index of about 3.6, the critical angle is about 16.degree., and thus, only 1.4% of the light emitted from the LED is obtained outside the crystal. Moreover, if an optical fiber for receiving the light emitted from the LED is of a plane-ended type, only incident rays within a acceptance angle on the plane-ended surface can be coupled into the fiber. Therefore, only about 2 percent of the divergent rays emitted from the plane surface of the crystal is coupled into the fiber.
In order to increase the quantity of rays coupled into an optical fiber, a so-called spherical-ended fiber, the end of which is manufactured as a sphere, is used. However, there is a problem in that such a spherical-ended fiber is expensive. Also, in order to increase the quantity of rays coupled into the spherical-ended fiber, it is necessary to match the position of spherical end with the emitting surface of the LED, within an accuracy of several micrometers. However, it is very difficult to do so, especially when LEDs are arranged in an array. The term "launching efficiency" used hereinafter is defined as the ratio of the incident optical power coupled into a fiber to the optical power generated in an LED.